Heat exchange method



Oct. 28, 1952 c. c. BRUMBAUGH 2,615,688

HEAT EXCHANGE METHOD Filed March 28. 1950 2 SHEETSSHEET 1 W7 INVENTOR. CHESTER C. BRUMBAUGH Patented Oct. 28, 1952 Chester .0. Brumbaugh, Painesvill'e, 0hio, assignor I to Diamond Alkali Company, Cleveland, Ohio, acorporation of Delaware Application March 28, 1950, Serial No. 152,314

This invention relates to heat exchange apparatus and to methods for operating such apparatus. It has particularly to do with start-up operations of heating, evaporating or concentrating apparatus of the surface heating typefor heat treating liquids in which heating tubes are positioned in place in apertured tube sheets, outer surfaces of such'tubes being exposed to a heating medium while inner surfaces receive liquid to be treated. I The invention is especially concerned with maintenance of 'the'seal between-tubes and tube sheets during start-up operations.

Especially in concentrators or evaporators, where a very substantial temperature differential exists between the shell side of the tubes and the material to be evaporated inthe tubes, a serious problem of thermal contraction'in the apparatus exists particularly on start-up, and this problem is aggravated at the point of seal between the tubes and the tube sheet. .Many concentration :or i evaporation systems incorporating. sheet and-tube heat exchangers have failed because of neglect'to take into accountthis very diificulty or of inability to solve the same, whereby leaks of'heat exchange fluid or vapor between the tubeand the tube sheet :have rendered the operation'of the apparatus :uneconomical. This phenomenon is especially observableinthe case where the tubes are-rolled intothe tube sheet and'the rolled joints between the tube and the tube sheet are initially ruptured at start-up, since the seal between :the tubes and sheet never re-establishes in the course of operation and completely taking down the evaporator and rebuilding the same often becomes unavoidable.

An object of thepresent invention is to provide :a .heat exchanger particularly adapted to heat materials at relatively high temperatures and at 'a'relatively great temperature differential between the material entering the heat exchanger 1 tubes and the'heat exchange medium, which apparatus comprises tubes and tube sheets, and to provide means for maintaining a tight joint between said tubes and tube sheets at start-up of theheat exchanger and in continuous operation thereof.

A further object of the invention is toprovide "a'method "for operating tube and sheet heatexchangers particularly on start-up, which method :is characterized by preservation of the joints be- "tweenthe tubes and the sheets of said heat exchanger. g l Afurther object is to provide a method of start- 2 Claims. (Cl; '257224) medium in contact with parts,-especially "joints between parts, which are subject to wide temperature difierent'ial during startup operation,

"whereby 'the d eleterious effects or thermal con- "traction especially 'at 'joints between tubes and sheets are mitigated.

A further object of the invention is' toprovide means for alleviating the thermal contraction brought about bythe relatively cool 'material'f-to be heated on'the tubes, which contraction occurs when the material'first comes in contact-withthe tubes which are-already at or nearthe temperature of the heat exchange fluid on the shell-side of the tubes.

A particular object of the'invention is to "pro-' vide a heat exchanger comprising an evaporator for concentration of fluids, which shall include .such means for mitigating injurious thermal contractionon start-up. Further objects and advantages will appear from the detailed descriptionwhich followsihereinafter and whichv includes thedrawings, inw-hich I ;Fig..1.is a side elevation,.partly in section,.showing I asheet ,and tube evaporator. and particularly illustrating the-relation-ofthe tubes to the tube sheet, J I

Figs. 2, 3,-and 4 are diagrammatic showings of various steps in prior art-start-up procedure,

parts being exaggerated for clarity of illustration,

and

Fig. .5ris a vertical longitudinal sectional ".view

7 of a different type of heat'exchanger having the principles of the invention applied thereto.

The present inventionmay specifically be considered in connection with asheet and tube evaporator particularly adapted ,for concentration of caustic soda'solutions, such as the 50% 01-73% solutions of commerce, toanhydrous caustic, em-

.ploying on the :shell side of the ':sheet and "tube evaporator a high temperature vapor heat :ex-

change medium, which is desirably .diphenyl or diphenyl 'oxide, especially an eutectic :mixture of 2 65% diphenyl 'and 73.5% 'diphe'nyl oxide :which hasra melting'point .of 53.6,F. and a normal boiling point of .500" F. and is known'by the tradenameDowtherm A. l igxlillustrates a formof up-of sheet and tube heat exchangers which ineludes maintaining an "auxiliary heat exchange this application. It will be appreciated, however, that the method of the invention and the apps.-

' 'ratus hereof may conveniently "be appliedwith substantially no modification to heating ofnmany other materials, especially where a considerable temperature differential exists between the material to be heated and the heat exchange medium, whether liquid orvapor, onthe she11 3 side of the evaporator tubes. An example of such application is shown in Fig. 5.

Referring particularly to Fig. 1, an evaporator is shown including shell 2, lower tube sheet 4, which may be secured by any convenient means (not shown) to collector 6, upper tube sheet 8, supplementary tube sheet ID, the tube sheets 8 and I conveniently being secured to the assembly by means of bolts I2. The evaporator also includes tubes I4. Heat exchange fluids or vapors, such as Dowtherm A or the like, may be introduced as by nozzle I6 and withdrawn at nozzle l8. Vent l9 may be situated at or near the top of the shell for a purpose to be described. Materials of construction are suitably relatively temperatureresistant, corrosion-resistant metals, such as nickel or the like.

Caustic soda solution or the like may suitably be introduced through conduit 20 onto the top of supplementary tube sheet l0 and be introduced into tubes l4 when the solution level reaches to the top of the tubes. Secured to upper tube sheet 8, as by Weld 24, is dam ring 26, having pipe 28, for introduction of cooling fluid and pipe 30 for withdrawal of the same. Suitable supplementary dams or sleeves 32 surround bolts l2 to prevent leakage at the bolt holes in the sheet 8. Obviously, a plurality of entrance conduits 28 and exit conduits 30 may be employed if desired, one of each being shown for illustrative purposes.

Referring to Fig. 5, a modified aspect of the method of the invention and modified apparatus is shown. The heat exchange assembly of this form. includes shell 40, head 4|, and base 43, shell 40 having inlet and outlet apertures 42, 44 for entrance and egress of heat exchange fluid. Tubes 46, 48 are horizontally disposed and mounted in tube sheets 50, 52, 54, the joints between the tubes and the sheets preferably being rolled for reasons pointed out herein. The space bounded by sheets 52 and 54 and walls 56, 58, through which the tubes 46, 48 pass, is out of contact both with the heat exchange fluid and thematerial to be heated. In accordance with the present invention, entrance and egress means 60, 52 are provided to this space, through which air or other suitable temperature modifying fluid may be passed. Head 4| is provided with entrance and egress means 64, 66 for material to be heat-treated and may suitably be divided by diaphragm 68.

While the shell space is being brought to operating temperature, cooling fluid, which may be air or other gas or a suitable liquid, such as water, is introduced at pipe 60 and withdrawn at pipe 62. Upon the shell area reaching the desired temperature, material to be heated or evaporated, or mixtures of materials to be reacted under heat, may suitably be passed through the apparatus through entrance 64, tubes 46, base 43, tubes 48, and exit 65. As will be discussed more fully below, the maintenance of tubes 48 at or near the egress end thereof under the influence of auxiliary heat exchange material entering at entrance 60 is probably unnecessary, but especially since the apparatus is somewhat more versatile since either opening 64 or 66 may be employed for entrance of material to be treated, the present form of evaporated into, referring to Fig. 1, the tube I4 simultaneously with bringing the interior of the.

shell to operating temperature or bringing the assembly up to temperature and then introducing the material to be evaporated. A disadvantage of the former type of start-up operation is that a variable yield is obtained ranging, as in the case of treatment of 50% caustic soda solutions to obtain anhydrous caustic, all the way from slightly over 50% caustic at the initiation of the operation to ultimately anhydrous caustic when operating temperature is achieved. The disposal of this material presents the alternates of discarding the same, which of course results in waste of product, or recycling to the evaporator when operation temperature has been achieved, the latter alternate, however, being unsatisfactory as special recycling equipment is necessary. Moreover, since the problem is only presented on start-up, such equipment scarcely justifies its existence economically. The latter procedure of bringing the interior of the shell to operating temperature and then introducing material to be treated into the tubes, whereby the first product-from the evaporator is fully evaporated material, avoids the disadvantage of wasted or recycled product.

Until the present invention, however, the problem of thermal contraction at the upper tube sheet on introducing the relatively cool material into the tube I4, while the area within the shell 2 was at operating temperature, has been a serious one as separation of the rolled or other joint between the sheet and the tube could scarcely be avoided. No such problem, of course, is presented once both the tubes and the sheet have come to thermal equilibrium at a temperature intermediate'that of the relatively cool material flowing down the tube 14 and the relatively hot heat exchange material within the shell 2. The startup problem, however, remained and is graphically illustrated in Figs. 2, 3, and 4, wherein is shown the effect upon the tube sheet 8 and supplemental tube sheet l0 when operated without the benefit of the present invention. Thus, in Fig, 2, the heating up stage is shown, wherein the area below the tube sheet 8 within the shell is brought to temperature, in the case of anhydrous caustic concentration, of the order of 700 F., i. e., the vapor temperature of Dowtherm under pressure between 75 and pounds/sq. in. gauge. Upon shell temperature being achieved in accordance with the procedure described above, relatively cool caustic liquor, such as a 50% solution of caustic soda in water, is fed to the supplemental sheet In until the same reaches the level of the tops of the tubes, whereupon it starts to flow down the tubes. The caustic solution, in order that the same shall not be in voilently boiling condition at this point, is at a temperature of the order of 250 F. as it is introduced onto the sheet I 0 and thus, as it flows down the tubes and comes to the level of the seal between the sheet 8 and the tube, is scarcely significantly hotter than that temperature. Thus, there is an approximately 400 temperature diiferential between the hot sheet 8 and the relatively cool tube [4 thereagainst, whereby a sudden contraction of the tube results and the rolled joint 34 between the sheet and the tube is ruptured by the sudden contraction of the tube away from the tube sheet. The breaking of this joint results in loss of heat exchange medium with consequent economic disadvantage, and moreover, in general, tends to destroy the overall efllciency of the concentrator. It is true that in time the tube sheet will be cooled by flow of heat to the cold tube, thus contracting the tube sheet and tending to compensate for the diflerential contraction between the tube and the tube sheet. However, once the rolled joints between the sheet and the tubes are broken, they never reseal themselves entirely and a constant source of irritation is accordingly present in the leaking heat exchange medium. It can be visualized readily by those skilled in the art that since a welded joint between the tubes and the tube sheet would also be subjected to sudden .and objectionable strain, the present invention is also applicable to systems employing such joints. However, this invention is primarily concerned with preservation of rolled joints because of general preference for this type based upon ease of assembly and tube replacement as compared to welded joints.

In contrast to the difiiculties encountered in operations of the prior art, attention is again directed to Fig. l and the manner in which the structure of the present invention may be operated and the method hereof practiced. In startup, the entire unit is brought up to temperature by introduction of the heat exchange medium, such as Dowtherm vapor under appropriate pressure, through the nozzle l6 until the shell area is brought to operating temperature, 1. e., of the order of 700 F. During this period of time, water or other suitable liquid has been present on the top of sheet 8 and preferably may be continuously flowing thereon inwardly through pipe 28 and out pipe 30 for cooling purposes. 'Water is the preferred cooling medium because the wellknown high heat transfer from metal to water maintains the tube sheet and the tubes at thermal equilibrium.

Upon the shell space coming to temperature, caustic is introduced through pipe 20 onto the supplemental sheet l and when it reaches the level of tube ends, commences to flow down the insides of the tubes. The tubes, however, are maintained at the joint between the sheet 8 and the tubes at a temperature well below the operating temperature of the Dowtherm, and in fact within 50 F. of the temperature of the sheet 8. Accordingly, the rolled joint between the sheet 8 and the tubes 14 is preserved intact. After caustic solution flow is achieved, the tubes and sheet 8 at their point of joint are held in thermal equilibrium by the coolant flowing o-ver tube sheet 8 and a rupturing differential of temperature between the two no longer exists.

As an additional precaution against rupturing differential at the joint of sheet 8 and tubes 14, vent l9, which suitably provides for removal of non-condensibles from the heat exchange medium, in the case of Dowtherm, mainly phenol, may be maintained closed during start-up, whereby a cushion of non-condensible and thus relatively poor heat conducting vapor will inhibit the flow of heat to the sheet 8. The vent l9 may be tween the tubes and sheet outside's'aid shell with While there has been illustrated and described in detail an embodiment of the invention, the described method is not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible and it is further intended that each element or instrumentality recited in any or" the following claims is to be understood as referring toall equivalent elements or instrumentalities for accomplishing substantially the same results in substantially the same or equivalent manner, it, being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

l. The method of starting up a tube and sheet evaporator having tubes within a shell and a tube sheet joined to said shell and said. tubes, which includes the steps of contacting the joints bea relatively cool fluid heat exchange medium, heating the shell side of said tubes to the desired operating temperature above the temperature of said heat exchange medium at said joints, and

introducing into said tubes the material to be treated in said tubes, said material being introduced at a temperature substantially below the operating temperature of the shell side of said tubes, While maintaining said heat exchange medium in contact with said joints during the introduction of said material.

2. The method of starting up a vertical sheet and tube heat exchanger having tubes within a shell and upper and lower tube sheets joined to said shell and said tubes, which includes passing a heat transfer fluid upon the upper tube sheet outside said shell and in contact with the juncture of said tubes and said upper tube sheet, heating the interior of the shell to the desired operating temperature substantially above the temperature of said heat transfer fluid upon said upper tube sheet, and thereafter introducing material to be evaporated into the top of said tubes, said material being introduced at a temperature substantially below the operating temperature of the shell side of said tubes, while maintaining saidheat transfer fluid on said upper tube sheet at the juncture thereof with said tubes until said sheet and said tubes assume substantially the same temperature.

CHESTER C. BRUMBAUGH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date I 1,848,939 Dempsey Mar. 8, 1932 2,412,573 Fraser, Jr. Dec. 17, 1946 

